Plant SABATH Methyltransferases: Diverse Functions, Unusual Reaction Mechanisms and Complex Evolution

It has been a quarter century since salicylic acid methyltransferase, the founding member of the plant SABATH enzyme family, was discovered. From making methyl benzoate to synthesizing caffeine, from modulating auxins and gibberellins to participating in strigolactone biosynthesis, new biochemical functions of SABATH methyltransferases have continuously been revealed, with more than 20 substrates of diverse structures identified. Three-dimensional structures for several members of the SABATH family have been solved, rendering explanations of reaction mechanism and structural basis of substrate specificity. The biological functions of several SABATH genes from diverse plants were elucidated by analyzing transgenic plants with overexpression, reduced expression or knock-out mutants of respective SABATH genes. In the majority of sequenced plants examined, SABATH genes constitute a medium-sized family. Diverse catalytic functions of SABATHs have evolved largely via gene duplication followed by functional divergence. Some SABATH family members have a deep evolutionary origin; some others, while catalyzing an identical reaction, evolved more than once. Equally important, many members acquired lineage-specific functions. All these are indicative of the important contributions of the SABATH family to land plant evolution and adaptation.